Lithium bisoxalatoborate, the production thereof and its use as a conducting salt

ABSTRACT

The invention relates to the novel lithium bisoxalatoborate compound, and to a method for producing this compound, on the basis of a lithium compound, an oxalic acid or an oxalate, and a boron compound. The invention also relates to another production method on the basis of lithium boron hydride and oxalic acid, and to the use of lithium bisoxalatoborate as a conducting salt in lithium-ion batteries.

The subject matter of the invention is lithium-bisoxalatoborate,Li[(C₂O₄)₂B], two methods for the production thereof, and the use oflithium-bisoxalatoborate as a conducting salt in lithium ion batteries.

At present, lithium hexafluorophosphate (LiPF₆) is used as a conductingsalt in all commercial lithium ion batteries. This salt has thenecessary prerequisites for use in high-energy cells, i.e. it is easilysoluble in aprotic solvents, it leads to electrolytes having highconductivities, and it has a high level of electrochemical stability.Oxidative decomposition first occurs at potentials of >approximately4.5V. LiPF₆, however, also has serious disadvantages, which are mainlyto be attributed to its lack of thermal stability. In solution, adissociation into LiF and PF₅ takes place, even if only slight, whichcan lead to a cationic polymerisation of the solvent, caused by theLewis acid PF₅. Upon contact with moisture, caustic hydrofluoric acid isreleased, which, on the one hand makes handling more difficult, becauseof its toxicity and corrosiveness, and, on the other hand, can lead tothe (partial) dissolution of the transition-metal oxides (for exampleLiMn₂O₄) used as cathode material. In this way, the cycle stability ofthe respective electrochemical energy store is affected.

With this background in mind, intensive efforts are being made with theaim of developing alternative conducting salts. As such, lithium saltswith perfluorated organic radicals are being tested above all. Inparticular, lithium trifluoromethane sulphonate, lithiumbis(trifluoromethane sulphonyl)imide and the lithium methides, the mostfundamental of which is lithium bis(trifluoromethane sulphonyl)methide,are to be mentioned. These salts also have disadvantages, which hithertoprevented their use in commercial lithium batteries. The first-mentionedsalt does not give the electrolytes produced with it a sufficiently highconductivity. The last-mentioned salts admittedly have a conductivitywhich is equal to that of LiPF₆, but because of the costly productionmethods are not of interest commercially. Additionally, the imide has acorrosive effect on aluminium sheets, which are used as currentdiverters in many battery systems. Apart from this, because of the highfluorine content of the compounds, under unfavourable conditionsexothermal reactions with lithium are to be feared.

Lithium organoborates were tested as a further class of compound for useas a conducting salt. However, their use in lithium ion batteries wasnot seriously taken into consideration because of the low oxidationstability, the safety problems linked with the formation oftriorganoboranes as well as their high price.

The lithium borate complex salts [(R¹O)₂B(OR²)₂]Li described in DE19633027 A1 represent a substantial step forward. In this connection, R¹and R² are the same or different, R¹ and R² are, if appropriate,connected to each other by a single bond or a double bond, R¹ and R² maybe, individually or jointly, an aromatic ring from the group phenyl,naphthyl, anthracenyl or phenanthrenyl, which can be unsubstituted orsubstituted one to four times by A or Hal, Hal standing for fluorine orchlorine and A meaning alkyl with 1 to 8 C-atoms, which in turn can behalogenised one to four times.

A disadvantage of these compounds is, on the one hand, the stabilitiesof the non-fluorinated derivatives which, although improved, are in noway sufficient for the 3V systems required. Thus, for example, theunsubstituted lithium-bis[1,2-benzenediolato(2-)-O,O{grave over ( )}]borate(1-) decomposes when an anodic potential of 3.6 V is exceeded.This value lies clearly below that of the standard conducting salt LiPF₆(approximately 4.5V). As a result of increasing fluorine substitution ofthe organic radical, the oxidation stability rises to a value ofapproximately 4V for the perfluorated compound. However, these valuesare still lower than in the case of the standard salt LiPF₆. Thestability of the borates which are described, however, increases furtherbecause of a top layer formation during cyclisation, so that for somecompounds almost sufficient stabilities are achieved. The stablecompounds, however, have high molar masses (for example 378 g/mol forthe perfluorated catecholate compound). Also, the preliminary stagesrequired for the synthesis are not commercially available, but insteadhave to be produced in a costly way. Finally, compounds with CF bondsrepresent a potential safety risk, because they are notthermodynamically stable with respect to metallic lithium.

The underlying object of the invention is therefore to eliminate thedisadvantages of the prior art and to develop an electrochemicallystable lithium compound which has a good solubility in the aproticsolvents used by the battery industry, and also a method for theproduction thereof.

The object is achieved by the lithium compound lithium-bisoxalatoborate,Li[(C₂O₄)₂B], indicated in claim 1. The independent claims 2 and 11indicate two different methods for the production oflithium-bisoxalatoborate, claims 3 to 10 and 12 to 13 develop the methodfurther and claim 14 indicates a use of the compoundlithium-bisoxalatoborate.

Surprisingly, although it does not have any fluorine substituents,lithium-bis(oxalatoborate) has an excellent oxidation resistance. Thus,solutions of this salt in a mixture of ethylene carbonate (EC) and1,2-dimethoxyethane (DME) are stable up to a voltage of 4.6V.

Furthermore, the conductivities which can be achieved with the salt inaccordance with the invention are note worthy. Thus, a 0.56 m solutionin a 1:1 mixture of EC and DME has a conductivity of 10.3 mS/cm at roomtemperature. In the usual solvent mixture propylene carbonate (PC)/DME(1:1), the conductivity of lithium-bisoxalatoborate in the case ofdifferent concentrations was measured (FIG. 1). It can be inferred fromthe measurement results that with concentrations of up to 15% by weight,conductivities of up to 14 mS/cm are achieved (see FIG. 1). These valuesare at the same level as, or even above, the conductivities which can beachieved with LiPF₆. Thus, for 1 m solutions of LiPF₆ in dimethylcarbonate (DMC)/EC, 11.0 mS/cm is achieved.

The molar mass of 193.8 g/mol is admittedly approximately 27% above thatof the LiPF₆, but clearly below that of the borates described in DE19633027 A1. This is not problematic, however, because electrolytes withlithium-bis(oxalatoborate) are also sufficiently conductive at lowerconcentrations (for example approximately 0.5 mol/l).

The lithium-bis(oxalatoborate) is easily soluble in water and in manypolar aprotic solvents. In tetrahydrofuran (THF), approximately 42% byweight dissolves at 50° C. and approximately 30% by weight dissolves at23° C. It has a solubility of at least 15% by weight in diethyleneglycol dimethyl ether (diglyme) and mixtures of diglyme and carbonates.

According to thermogravimetry experiments, lithium-bis(oxalatoborate) isfully stable at up to approximately 300° C.

The lithium-bis(oxalatoborate) in accordance with the invention can beproduced by reacting a lithium compound, such as lithium hydroxide(anhydrous or the hydrate) or lithium carbonate or a lithium alkoxide,with oxalic acid or an oxalate and a boron compound, such as boron oxideor boric acid or a boric acid ester.

The reaction can be carried out in a solvent, but does not necessarilyhave to be.

Preferably, lithium hydroxide or lithium carbonate is reacted with astoichiometric amount of oxalic acid and a stoichiometric amount ofboric acid or boron oxide in water, for example:

The reaction of lithium oxalate with oxalic acid and boric acid or boronoxide in water is also preferred, for example:

The sequence in which the components are added does not play asignificant role. Preferably, oxalic acid is placed in an aqueoussolution and the calculated amount of lithium base is added, or lithiumoxalate is mixed with the 3-fold molar amount of oxalic acid.Subsequently, the calculated amount of boric acid or boron oxide isadded to this partially neutralised oxalic acid solution.

The reaction temperature lies in the range between 0 and 100° C.

After the end of dosing, the mixture is heated to 50 to 100° C. for atime and the water is then distilled off. When crystallisation begins,the pressure is slowly lowered. The final drying takes place whilststirring, at approximately 50 to 150° C. and <approximately 1 mbar.

A solid product is obtainable which is partially lumpy, granular orfine-crystalline solid depending on the drying unit which is selected.

In a variant of the production method in accordance with the invention,water is not necessarily added as the solvent. However, water forms as areaction by-product in different amounts. According to this variant ofthe method, it is provided that the starting materials are suspended inan organic solvent and the water which is released during the formationreaction is removed by azeotropic distillation. All solvents whichcannot be mixed with water or which can be mixed therewith to a limitedextent, which form a water/solvent azeotrope and have such a highvolatility that a subsequent product drying is possible, are suitablefor this process. Depending on the temperature and stirring conditionsselected, the reaction starts spontaneously or is initiated by theaddition of small amounts of water. The reaction temperature of theexothermic reaction lies between 0 and 150° C. The reaction mixture issubsequently heated to boiling temperature, the water of crystallisationand reaction water being removed by azeotropic distillation. Aromaticsubstances, such as benzene, toluene, xylene and ethyl benzene, areparticularly suitable for the course of the reaction and the azeotropicdehydration. Thus, for example, when toluene is used, the calculatedamount of water can be precipitated within a reaction of time ofapproximately 2 to 4 hours.

The product in accordance with the invention precipitates infine-crystalline, free-flowing form, completely anhydrous and with goodpurity. It is separated from the reaction solvent by filtration, washedwith an aprotic solvent (for example toluene or comparatively volatilehydrocarbons, such as hexane or pentane) and dried in a vacuum and/or atcomparatively high temperatures (50 to 150° C.).

Ethers which cannot be mixed with water, such as 2-methyltetrahydrofuran, for example, are also suitable to a limited extent. Inethereal solvents, however, the lithium-bisoxalatoborate is only formedin impure form, i.e. it subsequently has to be purified in a relativelycostly way by fractional crystallisation.

According to a further embodiment of the method in accordance with theinvention, the product in accordance with the invention can also beobtained starting from lithium alkoxides LiOR and boric acid estersB(OR)₃ (with R=methyl, ethyl). In order to do this, a lithium alkoxideis mixed with a boric acid ester, the corresponding lithium tetraalkoxyborate Li[B(OR)₄] presumably being formed. This reaction does notnecessarily require a solvent, but can be carried out in the presence ofa solvent. The reaction mixture is subsequently reacted with oxalic acidand the alcohol component which is released is removed by distillation.Ideally, those boric acid esters which release as much volatile alcoholsas possible are taken for this variant, i.e. the methyl compound orethyl compound:

The alcohol itself (i.e. methanol or ethanol) or an aprotic solvent,such as acetonitrile, can be used as the solvent. In this variant of themethod, the reaction temperature amounts to 0 to 100° C., the rangebetween approximately 20 and 70° C. being most suitable. Whenacetonitrile is used, then, after distillation of the alcohol which isreleased at normal or reduced pressure, the product in accordance withthe invention precipitates upon cooling, in the form of colourlesscrystals, which can be filtered off and cleaned by washing withacetonitrile or another volatile, aprotic solvent (for example hexane,pentane, diethyl ether).

In accordance with a further variant of the method, LiBO₂ as bothlithium compound and boron compound can be reacted together with oxalicacid to form the desired product:

In accordance with a further production method in accordance with theinvention, lithium-bis(oxalatoborate) can also be prepared in aproticmedia directly in fully anhydrous form. In order to do this, lithiumboro-hydride is reacted in a solvent in accordance with the followingreaction equation with two equivalents of anhydrous oxalic acid:

The reaction is advantageously carried out in a solvent in which LiBH₄has a certain solubility, for example in ethers such as tetrahydrofuran(THF). Particularly advantageously, those solvents which are commonlyused by the battery industry for the production of electrolytes are alsoused. In particular, polyethers, such as 1,2-dimethoxyethane, aresuitable. The reaction temperature is not of crucial importance. It islimited downwards by the viscosity, which rises as the temperaturefalls. On the other hand, however, it should not rise too high, in orderto avoid a reductive attack, possible in principle, of the hydride onthe oxalic acid or lithium-bis(oxalatoborate). In general, thetemperature range between 20 and 50° C. is most suitable. The course ofthe reaction can be followed simply by observing the formation of gas.

In the following examples, the subject-matter of the invention isexplained in greater detail.

EXAMPLE 1

Synthesis of Li[(C₂O₄)₂B] from lithium hydroxide, oxalic acid and boricacid in water with subsequent total evaporation.

252.14 g (2.00 mol) oxalic acid dihydrate and 23.94 g (1.00 mol)calcined LiOH were dissolved in 1500 g distilled water. The temperaturerose to approximately 30° C. and a clear solution formed as a result ofthe heat of neutralisation. Within 15 minutes, a solution of 61.83 g (1mol) boric acid in 1300 g water was then added (no visible exothermy).The solution was then concentrated by distillation at normal pressure.Within approximately 3 hours, 2165 g water were distilled off. Thebottom temperature thereby rose to 104.2° C.; crystals precipitated outof the colourless solution. A further 450 g water were distilled off,and the remaining suspension (411 g) was placed in a porcelain cup andput in a vacuum shelf dryer for complete evaporation. After vacuumdrying for 24 hours at 100° C., 184.8 g (95% yield) of a colourlessgranulate were obtained.

found theory mol, normalised mol, normalised % to B = 1 % to B = 1 Li3.68 1.06 3.58 1 B 5.4 1.00 5.58 1 C₂O₄ 85.6 1.95 90.8 2 NMR data: δ¹¹B(THF/C₆D₆): 7.70 ppm h_(1/2) = 28 Hz δ¹³C (THF/C₆D₆): 159.1 ppm

EXAMPLE 2

Synthesis of Li[(C₂O₄)₂B] from lithium carbonate, oxalic acid and boricacid in toluene with subsequent azeotropic water separation.

126.07 g of oxalic acid dihydrate (1.00 mol) and 30.98 g of (0.500 mol)99.8% boric acid were suspended in 600 ml toluene in a 2 l four-neckedflask with thermometer, Teflon-blade stirrer and water separator. Afterheating to 60° C., first of all approximately 5 g of Li₂CO₃ were addedusing a solids dosing bulb. Within half an hour, no significantformation of gas could be established. Thereupon, 3.63 g of H₂O wereadded with a syringe. The reaction now began immediately, with strongformation of gas (2 l in 5 minutes). Within 5 minutes, the remainingamount of Li₂CO₃ (in total 18.50 g{circumflex over (=)}0.250 mol) wasadded. 6.19 l of gas ({circumflex over (=)}251 mmol, 100%) were therebyformed. The reaction mixture was then heated to boiling point andrefluxed for 4 hours. Already after 20 minutes, 57.3 g of water({circumflex over (=)}81% of the theoretically expected amount) hadprecipitated. Because the solid was baking strongly, it was cooledbriefly and the reaction mass was scraped off the flask wall with aspatula. After 4 hours of refluxing, no more water precipitated (intotal 72.0 g{circumflex over (=)}101% of the theoretically expectedamount); the yellowish suspension was cooled and filtered using a glassfrit. The cream-coloured, fine-crystalline sediment was washed twicewith hexane and first dried at room temperature to constant weight (97.4g{circumflex over (=)}100.5% of the theoretical yield). As result ofvacuum drying for 4 hours at 90° C., 0.2 of residual moisture wasremoved.

Analysis:

EXAMPLE 3

Synthesis of Li[(C₂O₄)₂B] from lithium hydroxide, oxalic acid and boricacid in toluene with subsequent azeotropic water separation.

8.70 g (125 mmol) of B₂O₃ (dried at 300° C. in a drying pistol) and63.04 g of (500 mmol) oxalic acid dihydrate were suspended in 300 ml oftoluene in a 500 ml four-necked flask with KPG stirrer, water separatorand thermometer. With the addition of 10.37 g (250 mmol) LiOH.H₂O, thetemperature rose spontaneously to 39° C. The azeotropic water separationbegan immediately after the boiling point was reached, and within 160minutes delivered 30.2 g of water ({circumflex over (=)}96% of thetheoretically expected amount). Because the reaction product stuck tothe flask wall, it was twice cooled slightly and the product was scrapedoff with a spatula.

Yield: 49.9 g of beige powder{circumflex over (=)}103% of thetheoretical yield.

EXAMPLE 4

Synthesis of Li[(C₂O₄)₂B] from lithium carbonate, oxalic acid and boricacid in 2-methyl tetrahydrofuran (2-MeTHF) with subsequent azeotropicwater separation.

252.14 g of oxalic acid dihydrate (2.00 mol) and 61.83 g of boric acid(1.00 mol) were suspended in approximately 0.8 l of 2-MeTHF and heatedto 40° C. in the same apparatus as in Example 2. 36.95 g (0.50 mol) ofLi₂CO₃ were then added in small amounts. To accelerate the reaction,2×1.5 ml water was sprayed in. The formation of gas took placerelatively evenly and produced approximately 255 mmol within one hour.Refluxing was carried out thereupon, for 13 hours. After 5 hours, thetheoretically expected amount of gas had escaped; the solution wasintensely yellow in colour and a total of 120.6 g of 2-MeTHF-saturatedwater precipitated in the water separator ({circumflex over (=)}114.2 gof pure water{circumflex over (=)}83% of the theoretically expectedamount). After 14 hours' reaction time, the yellow suspension was cooledand filtered by way of a G3-frit.

Analysis of the filtrate:

Filtrate: 1221 g, intensely yellow

NMR data: δ¹¹B (2-MeTHF/THF): 20.4 ppm h_(1/2) = 205 Hz 24% 7.66 ppm Li[(C₂O₄)₂B] 65% 5.25 ppm h_(1/2) = 72 Hz 11%

The product was subsequently freed from the solvent and crystallised outof THF/diethyl ether.

Yield: 83.3 g{circumflex over (=)}43% of the theoretical yield

Analysis of the product: the crystallisate dissolved in THF now onlyshows the ¹¹B-NMR-signal at 7.7 ppm

EXAMPLE 5

Synthesis of Li[(C₂O₄)₂B] from lithium methoxide, oxalic acid andtrimethyl borate in methanol.

4.97 g of (131 mmol) lithium methoxide were dissolved in 119 g ofmethanol, and at 30° C., within 10 minutes, and there was mixing with asolution of 13.51 g of (130 mmol) trimethyl borate in 30 g of methanol.The internal temperature thereby rose to 37° C.; the reaction solutionwas clear and colourless. 23.40 g (260 mmol) of anhydrous oxalic acidwere added to this solution all at once. The reaction mixture thereuponbriefly turned curd-like (approximately 10 seconds), in order then toturn into a slightly viscous, milky suspension. No exothermy could beestablished. The reaction mixture was boiled at reflux (66.6° C.) for 45minutes and, after cooling, was decanted from an extremely finelydispersed soft solid (the solid could not be separated with a G 3 g lassfrit). The total evaporation of the clear decanted solution on therotation evaporator produced 23.71 g of a greasy solid. Taking intoaccount the decantation loss, this corresponds to 25.4 g{circumflex over(=)}101% of the theoretical yield. In the rotation evaporator, smallamounts of a colourless sublimate were observed, which did not produce a¹¹B-NMR signal and dissolved in water with an acidic reaction, whichpoints to oxalic acid. The soft drying residue was not completelysoluble in THF. The THF-soluble portion, however, only showed a ¹¹B-NMRsignal at 7.7 ppm, which comes from Li[(C₂O₄)₂B]. The residue wasdigested with the approximately 6-fold amount of THF, filtered andevaporated. During evaporation, a greasy product resulted, which becameincreasingly dark in colour. After separation of the solvent, acolourless solid began to sublime off.

Yield (partly oily): 16.8 g ({circumflex over (=)}67% raw product)

The raw product was subsequently cleaned by recrystallisation out fromTHF/diethyl ether.

Yield: 10.2 g{circumflex over (=)}40% of the theoretical yield.

EXAMPLE 6

Synthesis of Li[(C₂O₄)₂B] from LiBH₄ and oxalic acid in THF.

68.06 g (0.756 mol) of oxalic acid, dried at 120° C. for two hours, weredissolved in 120 g of THF and cooled to −5° C. in a 0.5 l double-casingreactor. A solution of 8.10 g of LiBH₄ (0.372 mol) in 49.2 g of THF wasadded to this solution within 70 minutes. 22.6 l of gas (0.93mol{circumflex over (=)}63% of the theoretically expected amount) werethereby given off. It was then quickly heated to boiling point.Approximately a further 8 l of gas thereby escaped. After 45 minutes'boiling at reflux (66° C.), it was cooled to 24° C., a sample was takenand 3.3 g of LiH were added. 2.81 l of gas ({circumflex over (=)}116mmol) were given off. The suspension was filtered, with 300.3 g of clearfiltrate precipitating. The filtrate was then evaporated on the rotationevaporator to constant weight. 47.6 g (66% of the theoretical yield) ofa white powder were obtained, which for the purpose of purificationstill had to be recrystallised.

Analysis:

NMR data:

δ¹¹B (sample before lH addition): 9.7 ppm (32%); 7.7 ppm (68%)

δ¹¹B (filtrate before evaporation): 9.7 ppm (7%); 7.7 ppm (88%); 5.2 ppm(5%)

What is claimed is:
 1. Lithium-bisoxalatoborate, Li[(C₂O₄)₂B].
 2. Methodfor producing lithium-bisoxalatoborate, Li[(C₂O₄)₂B], wherein a lithiumcompound is reacted with oxalic acid or an oxalate and with a boroncompound.
 3. Method according to claim 2, wherein the reaction iscarried out in a solvent.
 4. Method according to one of claim 3, whereinthe solvent is water or an alcohol with 1 to 5 C atoms or an organicsolvent which cannot be mixed with water or can be mixed therewith to alimited extent and which forms an azeotrope with water.
 5. Methodaccording to claim 3, wherein lithium hydroxide or lithium carbonate orlithium oxalate is reacted with oxalic acid and boric acid or boronoxide in the ratio Li⁺:oxalate:B³⁺=1:2:1 and in the presence of water.6. Method according to claim 3, wherein lithium hydroxide or lithiumcarbonate or lithium oxalate is reacted with oxalic acid and boric acidor boron oxide in the ratio Li⁺:oxalate:B³⁺=1:2:1, an organic solventwhich forms an azeotrope with water is added to the water-containingreaction mixture and the water is removed azeotropically.
 7. Methodaccording to claim 6, wherein the organic solvent which forms anazeotrope with water is benzene, toluene, xylene or ethyl benzene. 8.Method according to claim 2, wherein the lithium compound is LiOH orLiOH.H₂O or Li₂CO₃ or lithium oxalate or LiOR, wherein R is methyl orethyl.
 9. Method according to one of claim 2, wherein the boron compoundis boron oxide B₂O₃ or boric acid H₃BO₃ or a boric acid ester B(OR)₃wherein R is methyl or ethyl.
 10. Method according to one of claim 2,wherein LiBO₂ is used as the lithium and boron compound.
 11. Method forproducing lithium-bisoxalatoborate, Li[(C₂O₄)₂B], wherein LiBH₄ isreacted with oxalic acid in an aprotic solvent.
 12. Method according toclaim 11, wherein the aprotic solvent is an ether or a polyether. 13.Method according to claim 12, the ether is tetrahydrofuran (THF) or thepolyether is 1,2-dimethoxyethane.
 14. In a lithium battery, theimprovement comprising the use of lithium-bisoxalatoborate,Li[(C₂O₄)₂B], as a conducting salt in lithium batteries.